EP0961285A1

EP0961285A1 - Device and method for reading nonvolatile memory cells

Info

Publication number: EP0961285A1
Application number: EP98830333A
Authority: EP
Inventors: Giovanni Campardo; Rino Micheloni; Alfonso Maurelli
Original assignee: STMicroelectronics SRL
Current assignee: STMicroelectronics SRL
Priority date: 1998-05-29
Filing date: 1998-05-29
Publication date: 1999-12-01
Anticipated expiration: 2018-05-29
Also published as: EP0961285B1; JP2000057789A; JP4237337B2; DE69820594D1; US6181602B1

Abstract

The reading method comprises the steps of: supplying simultaneously two memory cells (F1, F2), both storing a respective unknown charge condition; generating two electrical quantities (Va, Vb), each correlated to a respective charge condition; comparing the two electrical quantities (Va, Vb) with each other; and generating a two-bit signal (01, 02) on the basis of the result of the comparison. The reading circuit comprises a two-input comparator (58) comprising two branches in parallel, each branch being connected to a respective memory cell by a current/voltage converter (41). Both the two-input comparator (58) and the current/voltage converter (41) comprise low threshold transistors (49, 50, 65-68).

Description

The present invention relates to a device and a method for reading nonvolatile memory cells.

As is known, memory cells are presently read by converting the current flowing into the cell, which is suitably biased, into a voltage, and comparing the voltage thus obtained with a reference voltage generated from a reference cell, the charge state of which is previously known, and is typically a virgin cell. In fact, the read memory cell conducts a different current according to the stored charge condition, and comparison (carried out by a sense amplifier) with the current flowing in the reference cell allows detecting whether the cell is written or erased, and thus whether the stored datum is a "0" or a "1".

Figure 1 shows a simplified diagram of a reading device (sense amplifier) 1 connected to an array cell 2 to be read, and to a reference cell 3. Sense amplifier 1 comprises a circuit 4 preventing phenomena of soft-writing (spurious writing of the cell), a current-voltage converter 5 and a comparator 6.

Cell read correctness therefore depends to a large extent on the satisfactory operation of the reference cell.

At present in EPROM memories, reference cells are formed inside the memory array, using a column of the array as a reference, one for each output. This solution has some advantages, such as low dispersion of the threshold values of the reference cells compared with the values of the memory cells; simplicity of timing, since the reference cells are biased together with the memory cells; and balance of the branches of the sense amplifier.

However, this solution cannot be applied to flash-type memories, in which it is necessary for the reference cells to have a ground separated from that of the memory cells, to prevent the reference cells from becoming depleted (i.e. overerased) during memory cell erasing (which takes place in sectors). In addition, in flash-type memories, the arrangement of the reference cells inside the memory array would cause stresses for the reference cells themselves, such as to cause cycling problems and to prevent modification of the reference threshold if necessary during the test step, owing to the large number of reference cells. Consequently, in flash memories, the reference cells are gathered in a small array arranged outside the memory array. Thereby, the reference cells can be erased and/or written during the test step, to obtain the best reference possible, which nevertheless is the same for all the sense amplifiers.

In addition, a feature which is essential to obtain correct reading of the memory cells concerns positioning of the characteristic of the reference cell (reference characteristic), compared with the characteristics of written and erased memory cells, taking into account their distribution. In particular, with reference to figure 2, the position of the reference characteristic must be intermediate between the characteristic of the worst erased array cell (curve I_E, with threshold Vtc) and the characteristic of the worst programmed cell (curve I_W, with threshold Vts). To this end, known I/V converters are structured according to two solutions, i.e. unbalance converter, which provides the reference characteristic R₁ of figure 2, and semi-parallel converter which provides the reference characteristic R₂ of figure 3.

The two solutions have different fields of application; the first, of figure 2, is suitable for memories operating at high supply levels (5V); the second, of figure 3, is suitable for memories operating at a low voltage (less than 3V).

In these converters, the main problems are derived from the need to correctly position the reference, and to select accurately the gain of the trans-characteristic of the cell (gain seen externally), by modifying loads of the I/V converter 5. In fact these operations are very delicate and costly as to time; in addition, the reference cell (or plurality of reference cells) is not representative of the entire distribution of the array cells, and thus gives rise to a response distribution by the sense amplifier. Finally, the reference cells do not age like the array cells because they are subject to different stresses, they do not undergo the same program/erase cycles as the array cells, and on the other hand they are biased substantially continually during reading.

Consequently, design and control of the reference cells is difficult and complex.

The object of the invention is thus to overcome the above described disadvantages.

According to the present invention, a device and a method for reading nonvolatile memory cells are provided, as defined respectively in claim 1 and claim 11.

In practice, the reading device according to the invention does not use particular reference cells, having a previously known charge state, but compares with each other two bits read simultaneously, and preferably two bits of a single byte, using them as a dynamic reference for each other.

For understanding the present invention, a preferred embodiment is now described, purely by way of non-limiting example, with reference to the attached drawings in which:

figure 1 shows the circuit diagram of a sense amplifier of known type;
figures 2 and 3 show the characteristics of memory cells and reference cells in two known current/voltage conversion solutions;
figure 4 shows a simplified circuit diagram of the reading device according to the invention;
figure 5 shows a cross-section through a portion of a semiconductor material wafer accommodating a component of the diagram of figure 4; and
figures 6-8 show plots of electrical quantities measured on the circuit of figure 4, in three different reading conditions.

In figure 4, the reading device, indicated generally at 10, has a first and a second input node 11, 12, connected respectively to a memory cell F1 and F2, and a first and a

second output node

13, 14 providing respectively

output voltages

01 and 02. Cells F1, F2, of nonvolatile type, and in particular of flash type, are preferably cells of a single byte that are read simultaneously, and are biased at their gate terminals by reading voltages V_R, which, if the supply voltage has a sufficient value, has the same value as supply voltage Vcc, and otherwise is a boosted supply voltage by an appropriate circuit, in a per se known manner, not discussed in detail.

Each input node 11, 12 is connected by a respective

fedback cascode circuit

17, 18, to an

input node

19a, 20a of a respective first

current mirror circuit

19, 20.

Fedback cascode circuits

17, 18 comprise each an NMOS transistor 21, arranged respectively between

nodes

11, 19a, and 12, 20a, and an inverter 22 arranged respectively between nodes 11, 12 and the gate terminal of respective NMOS transistor 21.

Fedback cascode circuits

17, 18 regulate the voltage present on input node 11, 12, so as to prevent soft-writing phenomena, in a known manner. First

current mirror circuits

19, 20 comprise a PMOS transistor 23, diode-connected between nodes 19a, respectively 20a and a supply line 30 set to Vcc, and a transistor 24 connected between supply line 30 and a

respective output node

19b, 20b.

Transistors

23 and 24 have gate terminals connected to each other.

Output nodes

19b, 20b are connected by respective

fedback cascode circuits

31, 32, equal to

fedback cascade circuits

17, 18, to

input nodes

33a, 34a of respective second current mirror circuits 33, 34 of NMOS type, comprising

native transistors

35, 36, and thus have a threshold voltage that is lower than that normally provided. In particular, transistor 35 is diode-connected between the

respective input node

33a, 34a and ground 38; transistor 36 has a source terminal connected to ground 38 and a drain terminal forming the

respective output nodes

33b, 34b.

Output nodes

33b, 34b are connected by respective

fedback cascode circuits

39, 40 respectively to a first and a second input/

output node

41a, 41b of a current/voltage converter circuit 41.

Fedback cascode circuits

39, 40 are similar to

fedback cascode circuits

17, 18, except the fact that inverter 22 is replaced by a NOR gate 42, having a first input connected to node 33b, respectively 34b, and an other input receiving an enable signal EN supplied from the exterior. The output of NOR gate 42 is connected to a gate terminal of an NMOS transistor 43 arranged between node 33b, respectively 34b, and nodes 41a, respectively 41b. A first equalisation transistor 44 of NMOS type is connected between

nodes

33b and 34b, and has a control terminal receiving a signal ATD. A second equalisation transistor 45 of native NMOS type, is connected between gate terminals of NMOS transistors 43, and has a control terminal receiving signal ATD. In addition, a third equalisation transistor 46, of native NMOS type, is connected between the input/

output nodes

41a, 41b of current/voltage converter 41, and has a control terminal receiving signal ATD. Equalisation transistors 44-46 operate in known manner to equalise to each other the voltages present on

nodes

33b, 34b, and the voltages present on

nodes

41a, 41b in the equalisation step, when signal ATD (generated on detection of an address transition in the memory comprising the present reading device) has a high value, and are switched off during an actual reading step, such as to allow independent evolution of the two device branches, connected respectively to cell F1 and cell F2, and lead to input/

output nodes

41a, 41b, depending on whether cells F1, F2 are written or erased.

Current/voltage converter 41 comprises a pair of

load transistors

49, 50 of native NMOS type, diode-connected, and have a source terminal connected to the input node 41a, respectively 41b, a drain terminal connected to supply line 30, a gate terminal connected to the drain terminal, and bulk connected to the source terminal.

Load transistors

49, 50 are of triple-well type, as shown in the cross-section of figure 5, wherein the bulk of

load transistors

49, 50 is shown as comprising a P well 100 accommodating a source region 101 and a drain region 102 of N⁺-type. P well 100 is electrically connected to source region 101, and is accommodated in an N well 105 biased to Vcc, and in turn is formed in substrate 106, which is grounded. Thereby, the bulk is electrically separated from substrate 106, and has the same potential as source region 101; consequently

load transistors

49, 50 have a particularly low threshold voltage, which is not affected by the body effect (according to which the threshold voltage increases when the voltage drop between the body and source regions increases).

A

respective bias branch

51, 52 is arranged in parallel with each of the

load transistors

49, 50;

bias branches

51, 52 are equal to each other, and comprise a PMOS transistor 53 and a native-type NMOS transistor 54; PMOS transistor 53 has a source terminal connected to supply line 30, a gate terminal connected to ground 38, and a drain terminal connected to the drain terminal of NMOS transistor 54; NMOS transistor 54 has a gate terminal receiving signal ATD and a source terminal connected to the respective input/

output nodes

41a, 41b. During equalisation, when signal ATD is high,

bias branches

51, 52 initially set the flowing current, in a known manner, and maintain input/

output nodes

41a, 41b at voltage Vcc, less the threshold voltage of a native transistor.

First and second input/

output nodes

41a, 41b of current/voltage converter 41 are connected to a comparison circuit 58 comprising a first and a

second branch

59, 60 which are equal to each other and are arranged in parallel with each other. In detail, first branch 59 comprises a PMOS transistor 63 and three

NMOS transistors

65, 67, 69 connected in series between supply line 30 and ground 38; second branch 60 comprises a PMOS transistor 64 and three

NMOS transistors

66, 68, 70, also connected in series between supply line 30 and ground 38. NMOS transistors 65-68 are of native, low-threshold type; PMOS transistor 63 and

NMOS transistors

67, 69 of first branch 59 all have a gate terminal connected to the first input/output node 41a; PMOS transistor 64 and

NMOS transistors

68, 70 of second branch 60 all have a gate terminal connected to the second input/output node 41b.

NMOS transistors

65 and 66 of first and

second branch

59, 60 are diode-connected, have bulk connected to the respective source terminal, and are also of triple-well type, as

load transistors

49, 50.

PMOS transistors

63, 64 of first and

second branch

59, 60 have a source terminal connected to supply line 30 and a gate terminal connected to the drain terminal of

NMOS transistors

65, 66; the intermediate node between the

respective NMOS transistors

65, 67 and 66, 68, forms the first output 13 and, respectively the second output 14 of reading device 10; the source terminal of

NMOS transistors

69, 70 is connected to ground 38.

Under normal conditions, when cells F1, F2 belong to a same byte, four reading devices are necessary, with the same structure as the above described reading device 10, for reading the entire byte.

The circuit of figure 4 operates as follows.

Memory cells F1 and F2 both erased

In this case, even if two cells F1 and F2 absorb different currents, the current they absorb is mirrored in first and second

current mirror circuits

19, 20 and 33, 34. Then, at the end of the equalisation step, when signal ATD becomes low again, the voltage present on input/

output nodes

41a, 41b drops approximately to the threshold voltage of NMOS transistors 36 of second current mirror circuits 33, 34, which is very low (about 0.5 V) since NMOS transistors 36 are of native type. Consequently

PMOS transistors

63, 64 of comparison circuit 58 switch on, and

NMOS transistors

69, 70 switch off. In this condition,

PMOS transistors

63, 64 set both

outputs

13, 14 to a voltage having the same value as supply voltage Vcc, less the threshold voltage of

native NMOS transistors

65, 66 and thus

voltages

01 and 02 are both high, corresponding to a logic condition "11" (two-bit logic signal). This situation corresponds to the simulation of figure 6, wherein Va is the voltage present at the first input/output node 41a, Vb is the voltage present at the second input/output node 41b, and the other voltages have the meaning already explained.

Memory cells F1 and F2 both written

In this case, the cells do not absorb current, or absorb small currents, which may be also different from each other. Even in the worst conditions, the current absorbed by cells F1, F2, and mirrored in the first and second

current mirror circuits

19, 20 and 33, 34, is not sufficient to lower the voltage at input/

output nodes

41a, 41b, which in fact in ideal conditions goes to its maximum value, equal to supply voltage Vcc less the threshold voltage of native load transistors 49, 50 (0.5 V). Consequently

PMOS transistors

63, 64 of comparison circuit 58 remain switched off, and NMOS transistors 67-70 remain switched on.

Voltages

01 and 02 at

outputs

13, 14 are thus low, corresponding to a logic condition "00". This situation corresponds to the simulation of figure 7.

Cells F1 erased and F2 written

In this case, cell F1 conducts a high current, whereas cell F2 does not conduct current, or conducts little current. Consequently voltage Va at the first input/output node 41a is low, and voltage Vb at the second input/output node 41b is high; PMOS transistor 63 switches on,

NMOS transistors

67, 69 are switched off, PMOS transistor 64 remains switched off, and

NMOS transistors

69, 70 remain switched on. Voltage 01 at the first output 13 is therefore high, and voltage 02 at the second output 14 is low, corresponding to a logic condition "10". This situation corresponds to the simulation of figure 8.

Cells F1 written and F2 erased

This is a dual situation with respect to the just described one, which leads to logic condition "01".

Voltages

01 and 02 are subsequently advantageously buffered by a structure setting their value to a fully CMOS value.

In practice, with the described device, use of a comparison circuit comparing the content of two memory cells and supplying the result at the output as a two-bit signal, and use of native, low-threshold transistors in the current/voltage converter, as well as in comparison circuit, gives at the output an unambiguous binary signal coding all four possible states (written, erased), stored by two memory cells, the charge state of which is not previously known, unlike known circuits wherein characteristic and positioning of the reference cell or cells must be known accurately.

The advantages of the described device and the method are as follows. Elimination of the reference cell solves the above described problems of criticality, and difficulty of design and control; in addition it allows elimination of all the circuitry necessary for controlling and positioning the memory cell or cells in the EWS (Electrical Wafer Sort) step. Furthermore, it permits time saving in the EWS step, and, for reading an entire byte, requires the use of only four circuits as that described.

Finally, it is apparent that many modifications and variations can be made to the reading device and method described and illustrated here, all of which come within the scope of the invention, as defined in the attached claims.

Claims

A device (10) for reading nonvolatile memory cells (F1, F2), having two input nodes (11, 12) connectable to a respective one of two memory cells (F1, F2) storing unknown charge conditions, characterised by a two-input comparator circuit (58), said two-input comparator circuit having two inputs (41a, 41b), each connected to a respective one of said two input nodes (11, 12), comparing with each other said charge conditions, and generating at an output a two-bit signal (01, 02) coding said charge conditions.
A device according to claim 1, characterised in that said two-input comparator circuit (58) comprises a first and a second branch (59, 60) equal to each other, and connected in parallel between a first and a second reference potential structure (30, 38); said first branch (59) defining a first one (41a) of said two inputs and said second branch (60) defining a second one (41b) of said two inputs of said tow-input comparator circuit.
A device according to claim 2, characterised in that said first and second branch (59, 60) comprise each a first (63, 64) and a second (69, 70) transistor connected in series to each other, and of complementary type, said first and second transistor of each branch having gate terminals connected together and to a respective one of said first and second inputs (41a, 41b) of said two-input comparator circuit (58).
A device according to claim 3, characterised in that each branch (59, 60) of said two-input comparator circuit (58) additionally comprises a third (65, 66) and a fourth (67, 68) transistor connected in series between said first (63, 64) and second transistors (69, 70); said third and fourth transistors being of low-threshold type, and defining between each other an intermediate node (13, 14) forming a respective output of said two-input comparator circuit (58).
A device according to claim 4, characterised in that said first reference potential structure (30) is connected to a supply voltage (Vcc), in that said first transistor (63, 64) of said first and second branches (59, 60) is connected to said first reference potential structure (30), said third transistor (65, 66) of said first and second branches (59, 60) is diode-connected between said respective first transistor and a respective output (13, 14) of said two-input comparator circuit (58), and is of triple-well type.
A device according to any one of the preceding claims, characterised by charge detection means (17-54) arranged between said two input nodes (11, 12) and said two inputs (41a, 41b) of said two-input comparator circuit (58), and generates electrical quantities correlated to said charge conditions.
A device according to claim 6, characterised in that said charge detection means comprises a current/voltage converter (41).
A device according to claim 6 or 7, characterised in that said current/voltage converter (41) comprises two branches, each branch including at least one load transistor (49, 50) connected between a respective input node (11, 12) and a supply line (30); said load transistors being diode-connected and being of triple-well type.
A device according to claim 8, characterised in that each branch of said current/voltage converter (41) additionally comprises a first (19, 20) and a second (33, 34) current mirror circuit arranged between a respective input node (11, 12) and a respective load transistor (49, 50).
A device according to claim 9, characterised in that said current mirror circuits (33, 34) are connected to said respective load transistors (49, 50) and are of low threshold type.
A method for reading nonvolatile memory cells, comprising the steps of:

simultaneously supplying two memory cells (F1, F2), both memory cells storing a respective unknown charge condition;

generating two electrical quantities (Va, Vb), said electrical quantities being correlated to a respective one of said charge conditions;
characterised by the steps of:

comparing said two electrical quantities (Va, Vb) with each other;

generating a two-bit signal (01, 02) according to said comparison.